The present invention relates to generally to an improved suspension system for tilting vehicles. More specifically, the present invention relates to a hub arrangement for use in a tilting vehicle suspension that improves the overall performance of the vehicle by separating the tilting and steering axes in a manner that lengthens the tilting lever arm while shortening the steering lever arm in a manner that enhances overall vehicle control.
Currently, most motor vehicles on the highways are four-wheel vehicles that are typically larger, heavier and less fuel-efficient than three-wheel motor vehicles. Despite the fact that four wheeled vehicles are more common, there are several advantages to modern three-wheeled vehicles. For example, under most circumstances three-wheel vehicles by nature are more stable than four-wheel vehicles, as three points will form a plane under all circumstances, whereas four points do not. Another advantage is that three-wheel vehicles afford nearly ideal wheel loading for maximum tire traction in acceleration and braking situations. Despite the advantages that three wheeled vehicles enjoy over four-wheel vehicles, the main drawback of a three-wheel vehicle is that during a turn the centrifugal load of the vehicle is supported by only a single outside wheel rather than two, as would be the case in a four-wheeled vehicle. In this regard, the centrifugal force tends to overload the outside tire, causing the vehicle to slip out of the direction of the turn, unless some additional means of compensation is provided. Further, the three-wheel geometry allows the force vector associated with the vehicle's center of gravity to quickly fall outside the wheelbase of the vehicle causing an unstable condition whereby the risk of overturning the vehicle greatly increases. As the center of gravity in the vehicle rises higher, the potential for this to occur becomes much greater. In order to counteract these forces, generally three-wheeled vehicles employ a tilting-and-bank mechanism, which shifts the vehicle's center of gravity to the inside as it negotiated a turn thereby keeping more of the load on the inside tire and within the footprint of the wheelbase.
In the prior art, foot controlled tilting vehicle suspensions have typically employed ball joints placed at the four connecting points of a four-bar linkage to allow both vertical displacement for tilting as well as rotational displacement for steering. Such an arrangement creates several distinct problems. First, because the steering pivot must be positioned as close to the center of the wheel as possible, the trade off is that there is a greatly reduced distance of travel available for the foot controlled tilting function. As a result, a rider is required to apply a great deal of force with their feet to maintain the vehicle tilt through turns. Further, should a rider turn the vehicle too quickly, the force required to tilt the vehicle may increase so quickly that the rider may not be able to maintain the tilt, ultimately losing tilt control and flipping the vehicle.
Secondly, when using ball joints within a four bar suspension to allow both tilting and steering, the resultant steering axes are nearly vertical, greatly reducing steering performance and leading to bump steering effects for side mounted wheels. Ideally, steering axes for side-mounted wheels is configured in an angled fashion so that the axes intersect the tire contact patch, as evidenced by automobile suspension prior art. The problem is that this configuration is not possible if ball joints are used because the parallelogram four-bar linkage does allow proper vehicle tilting if it is wider at the bottom than at the top. Finally, the placement of ball joints at the ends of the linkage means that the linkage is not maintained in a rigid plane, except where it is pivotally connected to the frame. As a result, the connection at the frame must be extra strong to restrain any unwanted torsional articulation at the ends of the linkage members.
Accordingly, there is a need for a suspension system for a tilting vehicle that allows the forces generated by the vehicle to be transferred using a center of effort that is directed perpendicular to the traction surfaces of the tires. Further, there is a need for a suspension system for a tilting vehicle that provides for a separation of the tilting and steering axes to allow optimization of the forces needed for steering and tilting. Finally, the is a need for a suspension system for a tilting vehicle that both facilitates an increased leverage for foot control over the tilting function and provides an optimized positioning and geometry for steering control.